1. Field of the Invention
The invention relates to enclosed units, such as electronics or opto-electronics units, which liberate heat that must be rejected to the environment through the walls of the housing which encloses the unit. The invention has particular applicability to equipments having a plurality of units of this type which are mounted one above the other in a rack or cabinet (for simplicity, hereafter referred to simply as a rack).
An especially advantageous application of the invention is to units which contain one or more components which have a poor tolerance for elevated temperatures, and other components which liberate relatively large amounts of heat. It has long been known to locate the heat sensitive components away from and preferably lower than the hot, heat liberating components. However, when such units are mounted one above another in a rack, heat from the lower units tends to affect the units near the top of the cabinet adversely.
2. Description of the Prior Art
The use of integral fins to increase surface area and promote cooling goes far back in the age of machinery. Enough heat is liberated in machinery because of friction, thermodynamic or other processes so that some structure besides the basic machine parts was required to enhance cooling by the surrounding air. Devices like air compressors have long been made with fins cast onto the cylinders. Such fins generally extend radially and longitudinally, or radially in planes perpendicular to the cylinder axis.
When significant cooling is required, but it is desirable to rely on natural convection to provide flow of air over the fins, it was recognized very early that a vertical fin arrangement would provide the best air flow. However, for simplicity in manufacture, fins were generally aligned with the principal dimensions of the object.
With the advent of the electronics age, cooling of individual components became a problem. Copper oxide rectifier stacks were made with very large disc cooling fins between diodes of the stack. Some high power tubes were made with integral cooling fins, generally lying in longitudinally extending radial planes. With such tubes it was important that they be mounted in a vertical attitude.
With the development of power transistors, the first cooling improvement was radial fin devices that could be slipped over a transistor can. Later the higher power transistors were made such that the can was held against a large plate at the outside rear of an apparatus, and the plate had a number of vertical cooling fins on its exterior.
U.S. Pat. No. 4,237,521 teaches the use of a diagonal fin arrangement to facilitate cooling of a wall used as a heat sink for components such as transistors in a radio receiver. These transistors are generally mounted in intimate thermal contact with the heat sink. By arranging the fins obliquely to the vertical, adequate convection cooling could be provided for intermittent use; while for continuous use forced air cooling could be obtained by a fan placed behind the unit to withdraw heated air horizontally.
Another use of oblique fins is demonstrated in design U.S. Pat. No. 336,074. A CATV equipment housing is made of two cast sections, and has fins on all sides. On the two major walls the fins extend diagonally between the fins on the other sides of the housing.
A further improvement in cooling effectiveness is claimed for a fin design shown in U.S. Pat. No. 5,150,278. Here the fins are aligned in rows on one major surface having a long dimension, diagonal to the long dimension, and are interrupted by gaps which define angle channels oriented parallel to the long dimension, 30.degree. to 60.degree. from the row channels. This arrangement is described as providing improved air flow along the row channels because of reduced friction, and also allowing extra air flow along the angle channels. This housing is intended for mounting with the finned surface in a vertical plane, but the long dimension may be either vertical, such as when the unit is in a "pillar" enclosure, or horizontal, such as when the unit is hung from a wire stretched between poles. Cable connections to the unit are made at opposite ends, so that in a pillar mount one connector is in the top surface and the cable arches up away from the unit.
The CATV housing described in the '278 patent clearly is intended for use alone, so that no provision is made to preventing overheating of the top unit of a stack. If units, especially units like that shown in the '521 patent, are rack mounted, the only solution to high ambient temperatures around the top of a rack or cabinet has been to provide forced air cooling. This is disadvantageous because of cost, noise, and reliability considerations for small high speed fans.